THE IGNEOUS HOCKS. 499 



near the NAV. corner of sec. 10, T. 47 N., 1{. 27 W. (Atlas Sheet XXVIII). 

 lu other i)Uices some of tlie greenstones, where not schistose, are slightly 

 finer grained on their borders than in the intei-iors of the knobs, but in this 

 case the greenstone, near its contact with the rocks of the iron formation, 

 is highly charged with magnetite. Some of the magnetite is certainly titan- 

 iferous, like the most of the magnetite of tlie normal greenstone, but the 

 greater portion of it is nontitaniferous. The rock consists mainly of almost 

 amorphous chlorite. Scattered through this are large plates of a colorless 

 lamellar mineral that apjiears to be muscovite, and some grains of quartz. 

 A few long, columnar crystals of tourmaline, pleochroic in pink and very 

 dark bluish-green tints, are also scattered here and there among the other 

 components, but its presence is only doubtfully refen-ed to contact action, 

 as tourmaline has been found in small quantities in other greenstones both 

 of the eastern and of the western knobs. 



THE WESTERN KNOBS. 

 RELATIONS TO MARQUETTE SEDIiMENIS. 



As topogi-aphical features, the western knobs differ from the eastern 

 ones in that they are linear and dike-Hke in shape rather than irregular in 

 outline. The most typical of these knobs are in the area directly north of 

 Lake Michigamme, where they constitute Avell-defined hills rising boldly 

 as bare knolls above the general level of the surrounding country. Sim- 

 ilar greenstones occur also in the Republic trough, being best known at 

 Republic Alountain, where they are associated with the jaspers and schists 

 of the iron-bearing formation. At this place they are not higher than the 

 iron-bearing beds; hence they apjiear at first sight as interleaved sheets that 

 have been planed down by erosion equally with the iron-formation rocks. 



That the linear masses are intrusive rather than efiusive is shown by 

 the following facts: The knobs, while arranged in approximately straight 

 lines, are not continuous, but are separated from one another by little 

 valleys of sedimentary rocks; occasionally the individual knobs are not 

 confined to a definite horizon in the Marquette series, but cut across the 

 beds of a formation, or even cross the line between two contiguous forma- 

 tions and traverse parts of each, as is the case with the knob in sec. 21, 



